1. Field of the Invention
The present invention relates to a near-field-light-generating (NFL-generating) optical system for generating near-field light (NF-light) by receiving light. The present invention further relates to a head used for thermally-assisted magnetic recording in which a magnetic recording medium is irradiated with NF-light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
As the recording densities of magnetic recording apparatuses become higher, as represented by magnetic disk apparatuses, further improvement has been required in the performance of thin-film magnetic heads and magnetic recording media. In the magnetic recording media, it is especially necessary to decrease the size of magnetic grains that constitute a magnetic recording layer of the medium and to reduce irregularity in the boundary of record bit, thus to form minute record bits reliably. However, the decrease in size of the magnetic grains raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume. As a measure against the thermal stability problem, it may be possible to increase magnetic anisotropy energy KU of the magnetic grains. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. As a result, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field (coercive force) of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large magnetic anisotropy energy KU is used so as to stabilize the magnetization; anisotropic magnetic field of the medium is reduced by applying heat to a portion of the medium where data is to be written; just after that, writing is performed by applying write magnetic field (write field) to the heated portion.
In the thermally-assisted magnetic recording, a technique is well known, which utilizes a near field optical device (NF-optical device) as a metal piece that generates NF-light from plasmon excited by irradiated laser light. For example, U.S. Pat. Nos. 6,768,556 and 6,649,894 disclose a technique in which NF-light is generated by irradiating a metal scatterer with light and by matching the frequency of the light with the resonant frequency of plasmon excited in the metal.
As described above, various kinds of thermally-assisted magnetic recording systems with NF-optical devices have been proposed. Meanwhile, the present inventors have devised a NF-optical device in which laser light is coupled with the NF-optical device in a surface plasmon mode to cause excited surface plasmon to propagate to the opposed-to-medium surface, thereby providing NF-light, instead of directly applying laser light to a NF-optical device. The NF-optical device is hereinafter referred to as a surface plasmon generator. In the surface plasmon generator, its temperature does not excessively rise because laser light is not directly applied to the surface plasmon generator. As a result, there can be avoided a situation in which the end of a read head element, which reaches the opposed-to-medium surface, becomes relatively far apart from the magnetic recording medium due to the thermal expansion of the NF-optical device, which makes it difficult to properly read servo signals. In addition, there can also be avoided a situation in which the light use efficiency of a NFL-generating optical system including a NF-optical device is degraded because thermal fluctuation of free electrons increases in the NF light generator.
Here, the NFL-generating optical system is an optical system that includes a waveguide and a NF-optical device, and the light use efficiency of the NFL-generating optical system is given by IOUT/IIN (×100), where IIN is the intensity of laser light incident to the waveguide, and IOUT is the intensity of NF-light emitted from a near-field-light-generating end (NFL-generating end) of the NF-optical device after converting the laser light into surface plasmon in the NF-optical device.
To perform thermal-assisted magnetic recording in practice by using the above-described NFL-generating optical system including the surface plasmon generator, the end surface of the surface plasmon generator is required to be located as close to the end surface of a magnetic pole as possible in the opposed-to-medium surface, the magnetic pole being provided for generating write field. In particular, the distance between them in the direction along track is preferably set to 100 nm (nanometers) or less. Further, the distance between the emitting position of NF-light on the end surface of the surface plasmon generator and the write-field-generating position of on the magnetic-pole end surface is required to be set sufficiently small. By satisfying these conditions, there can be obtained a sufficiently large field gradient of write field generated from the magnetic pole in a position on the magnetic recording medium where NF-light is applied, thereby to enable write operations to be reliably performed.
However, generally, the NF-optical device is provided adjacent to the end portion on the opposed-to-medium surface side of the waveguide to convert the light propagating through the waveguide into NF-light. Here, the waveguide and the magnetic pole is required to be provided sufficiently apart from each other in order to avoid a situation in which the light use efficiency of the NFL-generating optical system is drastically reduced due to the absorption of the light propagating through the waveguide by the magnetic pole formed of a metal. This requirement conflicts with the requirement that the NF-optical device and the magnetic pole should be set as close as possible. Therefore, to resolve the conflict, important is the appropriate configuration and arrangement of the waveguide, the NF-optical device and the magnetic pole. Further, significantly important is to control the emitting position of NF-light on the end surface of the surface plasmon generator. Thus, it is understood to be significantly important that a NFL-generating optical system should be realized, in which a NF-optical device with an adjusted emitting position can be provided adjacent to the magnetic pole, in order to perform appropriate thermally-assisted magnetic recording.